Circuit for reducing static



Nov. 16, 1937. M. G. NICHOLSON, JR

CIRCUJvlT FOR REDUCING STATIC Filed Aug. 1, 1935 WE Si MQ mn EQ# Q Sow@mm INVENTOR WA( ATTORNEY Patented Nov. 16, 1937 PATENT oFFicE 2,099,311CIRCUIT FOR REDUCING STATIC Madison G. Nicholson, Jr., Verona, N. J.,assignor to National Union Radio Corporation, Newark, N.`J., acorporation of Delaware Application August 1, 1935, Serial No. 34,119

4 Claims. The present invention relates to a circuit designed to reducethe yeffect of certain forms of static which at present seriouslyinterfere with the reception of signals by a radio receiving set. Moreparticularly it contemplates the reduction o-r elimination of staticcrashes of the manmade type, such as automobile ignition noises, vacuumy,-vclea'ners, sewing machine motors, electric fans and similarelectrical devices. The nature of the disturbance from most of thesedevices is fundamentally the same as the static of long waves, that is,a pulse, or series of pulses, each of which lasts only a very shorttime, but due to their extremely great amplitude, they deafen thelistener to such a degree as to make a signal unintelligible.

In ordinary reception, the signal, which may be arriving at thereceiver, will, of the time, be undisturbed, while the other 5% of thetime the signal is blotted out by thestatic pulses. If the receivercould be made dead during this 5% of static pulses, the signal would bepractically unimpaired. Even if thesestatic pulsesy could be kept fromreaching amplitudes greaterA than that of the signal, it would still beeasily readable.

For many years it has been appreciated that a desirable circuit foraccomplishing this purpose would be one which would readily overload orbe blocked by these static crashes when they exceed the amplitude of thesignal. been used heretofore to limit static embodies the use of a verylow plate voltage on the last audio stage. 'This form of circuit,however, has several disadvantages, rst, the static crash is reduced butnot keptv down to signal level, mainly due to the large amount of audiofrequency harmonics generated; second, in order to utilize theadvantages to be derived it is necessary to maintain the signal voltageof the tube in the last audio stage atan amplitude just below that whichoverloads this stage.

The circuit contemplated by the present in vention is in principle thesame as the overloading audio amplifier but is carried on at radiofrequencies, and by the proper choice of circuit elements it is possibleto obtain a circuit which is' free from the first fault above mentioned.That is to say, the Vharmonicsv generated in the circuit by the staticcrashes are of such frequencies as not to be admitted, in anyappreciable magnitude, to the circuit presently to be described. In thecase of keyed carrier code the circuit o-f itself has inherentlyautomatic volume control characteristics over a fairly large signalrange such that thereis no trouble holding any signal which issufficiently strong to be received. Y

A circuit that has (Cl, Z50-20) The circuit organization which is thesubject of my invention may be applied to keyed carriers and modulatedcarriers. It will ber described in particular as incorporated in asuperheterodyne receiver. This form of the circuit may be said 5- toconsist of three parts: first, a diode detector having a resistor inseries, said detector being actuated by the last intermediate frequencystage in a superheterodyne receiver; second, a beat frequency oscillatorthe frequency of which differs 10 from the intermediate frequency by asuperaudible frequency; and, third, a radio frequency coupling devicesuch as a transformer, tuned to a frequency which is either the sum ordifference of the oscillator frequency andthe inter# 15 mediatefrequency, which feeds the audio detector.

My invention is illustrated schematically in the accompanying drawing,in which:

Fig. 1 is a' block diagram of a complete superheterodyne receiverincorporating the invention.

Fig. 2 illustrates schematically the diode detector'and oscillatorassociated therewith cornprising the static reduction circuit indicatedin block in Fig. 1.

' Fig. 3'is a graph showing the relation between input and outputvoltage of a diode translator.

Referring now to the circuit shown in Figure 2, it will be seen toembody a diode detector with a resistor, a second oscillator, input andoutput 30 transformers. Although this circuit is in realityA the essenceof my invention it is auxiliary in nature and its insertion or inclusionin a standard superheterodyne receiving circuit at the pointcontemplated by me accomplishes results 35 not obtained heretofore.

Such a circuit using the diode as a wiper tube Will have the followingcharacteristics: The output of this diode is proportional to the inputas long as the oscillator voltage is substantially greater than theinput voltage. On the other hand, if the signal voltage is larger thanthe oscillator voltage, the output is practically independent of theinput. y It is the latter characteristic around Which the presentinvention is based 45 and not the former. Figure 3 showsrthe foregoingcharacteristic graphically for oscillator voltage of 1.0, an outputcircuit with no losses, and a theoretical diode, that is, with nocontact potential'differences and with no curvature. In 50 the actualcase where the load does not have infinite impedance, nor the signalsource zero impedance, a resistor in series with the diode will tend tomaintain the impedance of the circuit constant, thereby obtainingresults more nearly 55 coinciding in shape with the curve of Figure 3even though the erhciency is reduced. Actually, the results obtained aresubstantially the same as those possible theoretically.

In the case of keyed carrier code reception, it will be apparent fromFigure 3 that there is less than 2 decibels change in output, for anychange whatsoever in the input, so long as it is greater than theoscillator voltage. This holds true for any type of signal or staticdisturbance. Therefore, if the signal voltage is maintained above thatof the oscillator a static disturbance can never be more than 2`decibels louder than the signal, even though static disturbancesreaching the receiver may have been or 80 decibels greater than thesignal.

For telephone reception (modulated carrier) it is desirable to have avery good automatic volumeV control, so as to operate the diode abouthalf way up the curve, as indicated at point A in Figure 3. If theautomatic volume control does not have sufficient control to hold thesignal to this point, two effects will be noted, namely, iirst, if thesignal level rises up on to the fiat portion of the curve, themodulation will be distorted, and the percentage of modulation willappear greatly reduced; second, if the signal level falls below thispoint, the signal reaching the audio detector will fall offproportionally as the operating point is moved down the curve, theresult is, ther signalto-noise ratio is poorer. If the actuating voltagefor the automatic volume control is taken oi before the signal passesthrough the static reducing circuit, loud static pulses will actuate theaud tomatic volume control making the signal disappear during a seriesof static pulses which are separated by a length of time less than thetime constant of the automatic volume control system. If the actuatingvoltage is taken off after passing through the static reducing circuit,the same difculty is encountered, but to a much less degree. This may bereduced still further by passing the actuating voltage through a veryselective circuit, before applying it to the automatic volume control.In certain special cases it is possible to place this sharply tunedcircuit between the static reducing circuit and the audio detector, inwhich case the actuating voltage is taken directly oi the audiodetector; if it is a diode detector, there is the possibility of takingthe direct current component and, by amplifying it, have the necessaryvoltage to control the grids of the radio frequency and intermediatefrequency tubes.

The action of the selective circuit in reducing the effect of staticpulses upon the automatic volume control tube is brought about in thefollowing way: these static pulses are composed of radio-frequencycomponents which cover wide frequency bands, usually megacycles wide,and with a rather uniform distribution. Since these are passed throughtuned circuits which, all together, have a band width of only a fewkilocycles, the resulting disturbance reaching the static reducingcircuit has a frequency distribution coinciding with the resonance curveof the receiver Vup to this point. The output of the diode has afrequency distribution similar to its input, but limited in amplitude tothe maximum which the circuit will pass. If this is passed through aselective circuit which has, for example, a band width of only one tenthof that of the circuits up to this point, the energy of the noise willbe reduced tenfold, while that of the signal will not be reduced at all,providing the signal does not have a band width greater than that of theadded very selective circuit. cuit is added for clearing up theactuating voltage for the automatic volume control tube, one is onlyconcerned with the carrier getting through and does not care if some ofthe Side bands are cut. With such a large ratio as ten, the voltagearriving at the automatic volume control tube would actually decreaseduring the time when a static pulse is arriving, which would result incausing the automatic volume control to tend to make the radio set moresensitive, just the opposite of the result if the selective circuit hadnot been added. A'selectivity ratio of two to one would probably givethe most constant automatic volume control action through static pulseswhere the diode is being operated half way up the characteristic curveshown in Figure 3.

vThe use of added selectivity, between the static reducing circuit andthe audio detector, will result in a better signal-to-noise ratio, beingroughly proportionalto the ratio of selectivities of the receiver andthe added circuit. This is true for both telephone and keyed carriercode; in the latter case much more selectivity canbe used since thetransmission is mainly connedvto a single frequency. As selectivity `isadded, the receiver approaches the condition of being silent duringstatic pulses, but with full sensitivity at all other times.

While the invention has been set forth and described in a preferred formnevertheless obvious modifications of the circuit arrangementcan bemade. It is desired to cover all such modifications as come within thescope of the appended claims.

What is claimed is: Y

1. In a radio receiving signaling system, a static reducing circuit,comprising a diode detector having a resistor in series, said detectorbeing actuated by the last intermediate frequency stage inYa'superheterodyne receiver, an oscillator the frequency of whichdiifers from said intermediate frequency by a superaudible frequency,and a radio-frequency coupling device tuned to a frequency which is theresultant of the oscillator and the intermediate frequencies, saidcoupling device feeding an audio detector.

2. In a radio receiving set, a static reducing circuit comprising adiode detector which is fed by the signal from the last intermediatefrequency stage in a super-heterodyne receiver, a beat oscillator whoseapplied voltage to said diode is approximately twice that of the voltageof the unmodulated signal from said last intermediate frequency stage,whereby said diode provides limiting action for noise impulses whoseamplitudes exceed twice said unmodulated signal voltage but stillproviding for normal modulation range, the frequency of said oscillatorbeing different from the intermediate frequency, and a radio frequencycoupling device tuned to a frequency which is the resultant of saidfrequencies which feeds an audio detector.

3. In a radio receiving system, a static reducing circuit, comprising adiode detector having a resistor in series, said detector being actuatedby a radio frequency signal, an oscillator the frequency of which dinersfrom the'frequency of said signal by a super-audible frequency, and aradio frequency coupling device tuned to a frequency which is theresultant of the oscillator frequency and the frequency of said signal,said coupling device feeding an audio detector.

4. In a radio receiving system, a static reducing circuit comprising adiode detector which is Where this cirfed by the last intermediatefrequency stage in a superheterodyne receiver, a beat oscillator Whoseapplied voltage to said diode is approximately twice that of the outputvoltage of said last intermediate frequency stage, whereby said diodeprovides limiting action for noise impulses whose amplitudes exceedtwice said unmodulated signal voltage but still providing for normalmodulation range, the frequency of said oscillator being different fromthe intermediate frequency, and a radio frequency coupling device tunedto a frequency which is the resultant of said frequencies which feeds anaudio detector.

MADISON G. NICHOLSON, JR.

